A key requirement for efficiently operating induction motors is to run them under ideal conditions, by providing optimum electrical drive waveforms using minimum power input while maintaining the desired speed. In order to do this it is essential to know the speed at which they are running and this has not always proven straightforward.
AC induction motors, that typically operate with a 50 Hertz (Hz) AC supply, are cheap to manufacture and reliable to operate. These motors are found in a myriad of industrial applications, such as for pumps, compressors, fans and drive systems. AC motors can be sub-divided into types according to the number of phases of the power supply. For example, AC motors may be single or three phase. Three phase AC induction motors tend to be more efficient than single phase AC motors. In recent years there has been growing legislative pressure to improve inefficient machines, reduce unnecessary energy waste and minimise so-called ‘carbon related emissions; i.e. from the perspective of their effect on environmental conditions. Pressure is in the form of lobbying, private pressure groups and also from increasingly stringent legislation and is particularly directed at electric motors.
Designers of electric motors and motor controllers have therefore concentrated on ways of improving the efficiency of motors and their controllability, with a view to operating them at optimum conditions so as to extract more useful power.
In an attempt to increase the efficiency of the ratio of electrical power in—to the useful power out, there is a trend to use Electrically Commutated Motors (ECM) instead of Induction Motors. Although EC motors do have some advantage in terms of efficiency compared with a standard induction motor, this comes at a price. In particular ECMs suffer from magnet desaturation if overloaded, thus resulting in an absolute torque limit. Also there is a tendency for the magnets to wear out over a period of time. Furthermore, motor cogging may occur due to the pulse nature of winding currents and also rotor position and rotation sensing elements may be required within the motor itself.
It should also be noted that the production and disposal of permanent magnets gives rise to potentially environmentally-damaging pollutants that require specialised handling and treatment. This raises issues for both their manufacture and end of life recycling.
One area of particular interest is AC motors that are rated at mains voltage (typically 230 volts), but driven with an input voltage of around 50-180 volts. One reason for this is that these motors are very common as they are used in many types of domestic equipment and industrial systems such as: domestic appliances, coolers, ventilation and air conditioners. Manufacturers are being encouraged both by legislation and the perceived benefits of adopting an environmentally-responsible attitude in an attempt to ensure they meet the increasingly stringent environmental demands.
Losses and inefficiencies of many AC induction motors were in fact due to the imbalance that often exists between the requirements of a mechanical load, the way a motor operates and the control and conditioning of available power. The net result is that in the past a significant amount of energy was wasted.
The exact speed of an induction motor is normally measured using external or internal components or devices such as a ‘tacho’, a Hall Effect device or an optical device, for example.
An approximate speed may also be calculated using theoretical mathematical models, for a particular make and size of motor with a particular load. The generalised extension of these models may have a significant level of inaccuracy.